[R-sig-ME] group-specific model slopes differ from the global slope: random slopes in lme?

[Tim Richter-Heitmann]

Dear List,

i am running lme with spatial correlation structures for a dataset of 
~180 observations across six groups:

fit <- lme(outcome~var1+var2+var3, random=1|group, correlation=cor...., 
data, method="ML").

For some models, this works well; plotting the data reveals a nice 
agreement within the six groups in regards of the slope of the model.

However, for other models, plotting the data reveals that the global 
slope is clearly differing from a group-specific slope (interestingly, 
the global model explains the data fairly well, when plotted without 
facetting by group, also the qqplots look nice).

I found some references in the net that lme can incorporate random 
slopes, but i have not found tutorials how to do this. Advice would be 
well appreciated

Also, philosophically, isnt a random slope, random intercept-model 
equivalent to modelling the six groups independently? Also, how to 
report this in a manuscript? Or better not use random intercept models 
at all?

-- 
Dr. Tim Richter-Heitmann

University of Bremen
Microbial Ecophysiology Group (AG Friedrich)
FB02 - Biologie/Chemie
Leobener Straße (NW2 A2130)
D-28359 Bremen
Tel.: 0049(0)421 218-63062
Fax: 0049(0)421 218-63069